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Nuclear energy applications

In a fission nuclear reactor, uranium-238 can be used to breed 239Pu, which itself can be used in a nuclear weapon or as a nuclear-reactor fuel supply. In a typical nuclear reactor, up to one-third of the generated power does come from the fission of 239Pu, which is not supplied as a fuel to the reactor, but rather, produced from 238U.

Breeder reactors

238U is not usable directly as nuclear fuel, though it can produce energy via "fast" fission. In this process, a neutron that has a kinetic energy in excess of 1 MeV can cause the nucleus of 238U to split in two. Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the about 1.7 neutrons produced in each fission have enough speed to continue a chain reaction.

238U can be used as a source material for creating plutonium-239, which can in turn be used as nuclear fuel. Breeder reactors carry out such a process of transmutation to convert the fertile isotope 238U into fissile Pu-239. It has been estimated that there is anywhere from 10,000 to five billion years worth of 238U for use in these power plants.[3] Breeder technology has been used in several experimental nuclear reactors.[4]

By December 2005, the only breeder reactor producing power was the 600-megawatt BN-600 reactor at the Beloyarsk Nuclear Power Station in Russia. Russia has planned to build another unit, BN-800, at the Beloyarsk nuclear power plant. Also, Japan's Monju breeder reactor is planned to be started, having been shut down since 1995, and both China and India have announced plans to build nuclear breeder reactors.

The breeder reactor as its name implies creates even larger quantities of Pu-239 than the fission nuclear reactor.

Downblending

The opposite of enriching is downblending. Surplus highly enriched uranium can be downblended with depleted uranium or natural uranium to turn it into low enriched uranium suitable for use in commercial nuclear fuel.

238U from depleted uranium and natural uranium is also used with recycled Pu-239 from nuclear weapons stockpiles for making mixed oxide fuel (MOX), which is now being redirected to become fuel for nuclear reactors. This dilution, also called downblending, means that any nation or group that acquired the finished fuel would have to repeat the very expensive and complex chemical separation of uranium and plutonium process before assembling a weapon.

The larger portion of the total explosive yield in this design comes from the final fission stage fueled by 238U, producing enormous amounts of radioactive fission products. For example, an estimated 77% of the 10.4-megaton yield of the Ivy Mike thermonuclear test in 1952 came from fast fission of the depleted uranium tamper. Because depleted uranium has no critical mass, it can be added to thermonuclear bombs in almost unlimited quantity. The Soviet Union's test of the "Tsar Bomba" in 1961 produced "only" 60 megatons of explosive power, over 90% of which came from fusion, because the 238U final stage had been replaced with lead. Had 238U been used instead, the yield of the "Tsar Bomba" could have been well-above 100 megatons, and it would have produced nuclear fallout equivalent to one third of the global total that had been produced up to that time.

Radium series (or uranium series)

The 4n+2 chain of 238U is commonly called the "radium series" (sometimes "uranium series"). Beginning with naturally occurring uranium-238, this series includes the following elements: astatine, bismuth, lead, polonium, protactinium, radium, radon, thallium, and thorium. All are present, at least transiently, in any uranium-containing sample, whether metal, compound, or mineral.

The mean lifetime of 238U is 1.41×1017 seconds divided by 0.693 (or multiplied by 1.443), i.e. ca. 2×1017 seconds, so 1 mole of 238U emits 3×106 alpha particles per second, producing the same number of thorium-234 (Th-234) atoms. In a closed system an equilibrium would be reached, with all amounts except for lead-206 and 238U in fixed ratios, in slowly decreasing amounts. The amount of Pb-206 will increase accordingly while that of 238U decreases; all steps in the decay chain have this same rate of 3×106 decayed particles per second per mole 238U.

Thorium-234 has a mean lifetime of 3×106 seconds, so there is equilibrium if one mole of 238U contains 9×1012 atoms of thorium-234, which is 1.5×10−11 mole (the ratio of the two half-lives). Similarly, in an equilibrium in a closed system the amount of each decay product, except the end product lead, is proportional to its half-life.

As already touched upon above, when starting with pure 238U, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, for one mole of 238U, 3×106 times per second one alpha and two beta particles and gamma ray are produced, together 6.7 MeV, a rate of 3 µW. Extrapolated over 2×1017 seconds this is 600 gigajoules, the total energy released in the first three steps in the decay chain.